Ultraviolet rays reaching the ground are known to include middle ultraviolet rays (290-320 .mu.m; called UV-B) which induce rash of the skin, followed by inflammation leading to skin darkening and are regarded as one of causes of cutaneous cancer, and near ultraviolet rays (320-400 nm; called UV-A) which are less causative of rash but oxidize reduced melanin in the skin to cause deposition of melanin pigment leading to skin darkening or which are regarded to accelerate aging of the skin through long-term exposure.
With elucidation of the influences of ultraviolet rays on the skin, there has been an increasing demand for sunscreening cosmetics. A sun protection factor (SPF) value has been proposed as an index for protection of the skin from ultraviolet rays, and a higher SPF value is deemed to indicate a higher sunscreening effect. Reference can be made, for example, to Fukuda Minoru, KOSHOKAISHI, "Sun-Screen and SPF," Vol. 8, No. 4, pp. 315-323 (1984).
In general, sunscreening cosmetics contain ultraviolet absorbents or ultraviolet scattering agents for obtaining high SPF values.
Known ultraviolet absorbents include benzophenone compounds, cinnamic acid compounds, and benzoic acid compounds. Achievement of effective sunscreening requires incorporating a relatively large quantity of the ultraviolet absorbent into cosmetic compositions. However, incorporating a large quantity of the ultraviolet absorbent causes various problems, such as reduction in compatibility with cosmetic bases, adverse influences on cosmetic bases (e.g., denaturation), decomposition of the ultraviolet absorbent which leads to reduction in ultraviolet absorptivity, coloring, etc., and impairment of safety (e.g., increased irritation to the skin). Therefore, use of ultraviolet absorbents as cosmetic material involves many disadvantages, and cosmetics containing such materials in large quantities lose their values as cosmetics.
On the other hand, ultraviolet screening agents include zinc oxide, titanium oxide, zirconium oxide, and kaolin. These inorganic pigments show an effect of screening a broad range of ultraviolet rays, are less liable to denaturation on exposure to ultraviolet rays, and are less irritating to the skin because they do not penetrate into the skin. However, when they are applied to the skin, they give an unnatural white look to the skin due to their high covering power.
In order to overcome these disadvantages, zinc oxide, titanium oxide, zirconium oxide, etc. in the form of fine particles have been proposed. However, such particles undergo secondary agglomeration when incorporated into cosmetics, not only failing to exhibit sufficient sunscreening effects as expected from sunscreening cosmetics, but also resulting in poor spreadability on the skin.
In the light of the above-described circumstances, plate-like metal oxides have been proposed for use as cosmetic materials. These materials are said to have high safety, provide a satisfactory makeup finish, and exhibit high ultraviolet screening ability, as disclosed in Japanese Patent Application Laid-Open Nos. Sho-61-161212, Sho-62-4212, Sho-63-166819, and Hei-1-175921.
Although these materials are safe and give a natural makeup finish, the sunscreening effects attained are still insufficient irrespective of whether they are used alone or in combination with ordinary ultraviolet absorbents in amounts that cause no safety problem.
Accordingly, there still has been a demand in the art to develop cosmetics that are safe to use and yet have high sunscreening effects.
An object of the present invention, therefore, is to solve the above-mentioned problems. That is, an object of the present invention is to provide a cosmetic which exhibits high ultraviolet-screening effects (sunscreening effects), which is excellent in safety, which stably maintains an ultraviolet absorbent in the base thereof, and which gives a natural makeup finish.